Switching power amplifier

ABSTRACT

Switching power amplifiers are used for example in audio applications. Using Class D technology is used to obtain efficient performance.  
     To improve the switching power amplifier further it is proposed to combine a plurality of low-power switching power stages into a larger amplifier. The precision and efficiency of a switching power amplifier is improved without deteriorating the noise and EMI.

[0001] The invention relates to a switching power amplifier for supplying an audio output signal.

[0002] From the European patent application EP-A-0930700 a so-called class D amplifier is known, which receives at an input a so-called PCM input signal.

[0003] Using traditional class D technology any of three imperfections, that is efficiency, linearity or EMI may be improved upon with the expense of at least one of the other two. The switching frequency is directly proportional to the required signal bandwidth. Further the high repetition rate of for example DSD (Direct Stream Digital) will translate into a vary high switching noise and thus low efficiency. A DSD signal is a representation of an audio signal using the following parameters. Sampling rate=64 or 48 times 44.1 kHz. One bit, noise shaped. Maximum modulation index=50%. Using bandwidth 60 kHz, spectral information up to 100 kHz.

[0004] It is an object of the invention to improve the known switching power amplifier.

[0005] To this end a switching power amplifier according to the invention comprises the features of the independent claim(s). The precision and efficiency of a switching power amplifier is improved by combining a plurality of low-power switching power stages into a larger amplifier.

[0006] In this way it is made possible to increase the efficiency of the switching power amplifier without deteriorating the other imperfections. Embodiments of the invention are described in the dependent claims.

[0007] The advantages of multiple phases are: output switches will need to supply only a fraction of the total power, the switches are smaller and hence switch faster, with less losses, the number of output transitions is distributed across these smaller switches, and the influence of a timing error on one switch will be 1/n of what it would have been with a single phase.

[0008] A specific drive method is used to allow for low-loss combination of the signal voltages. An inductive combiner is devised to provide error-free combination. This method allows for direct conversion of for example DSD signals into analog power with low distortion and high efficiency.

[0009] The invention and additional features, which may optionally be used to implement the invention to advantage will be apparent from and elucidated with reference to the examples as described here below and hereinafter and shown in the figures. Herein shows:

[0010]FIG. 1 block-schematically a switching power amplifier according to the invention,

[0011]FIG. 2 a block-schematically switching power amplifier according to the invention in more detail with a DSD input signal,

[0012]FIG. 3 a block-schematically example of a switching power amplifier according to the invention with a PCM input signal,

[0013]FIG. 4 block-schematically an analog implementation of a switching power amplifier,

[0014]FIG. 5 an example of an inductor summation means.

[0015]FIG. 1 shows block-schematically an example of a switching power amplifier SPA. At an input I the switching power amplifier receives an input signal. The input I is coupled to an input unit INU for converting the input signal into a signal which can be used in the multiphase drive unit MPDU. The output of the multiphase drive unit is coupled to the power stage PS. The output of the power stage is coupled to a summation unit SUMU. The output of the summation unit is coupled via a low pass filter to the output O of the switched power amplifier.

[0016] The advantages of multiple phases are: output switches will need to supply only a fraction of the total power, the switches are smaller and hence switch faster, with less losses, the number of output transitions is distributed across these smaller switches, and the influence of a timing error on one switch will be 1/n of what it would have been with a single phase.

[0017]FIG. 2 shows an example of a switching power amplifier SPA2 that receives at an input I2 a so-called DSD signal. The input I2 is coupled via the input unit INU2 to the multiphase drive unit MPDU2. Also in this example the multiphase drive unit is coupled to the power stage PS2. And the output of the power stage is coupled to the summation unit SUMU2. The output of the summation unit is coupled via a low pass filter LPF2 to the output O2 of the switching power amplifier.

[0018] The input unit INU2 filters in this example the input DSD signal, and supplies k signals to the multi phase drive unit MPDU2. The multi phase drive unit supplies n drive signals to the power stage PS2 that is in this example an n-phase power stage. The output of the power stage is coupled to a summation unit for summing the n different output signals. The output of the summing unit is coupled via the low pass filter LPF2, for example an LC low pass filter to the output O2 of the switching power amplifier SPA2.

[0019] This method of producing gain allows the DSD signal to modulate the amplifier fully, even if its modulation level is too low.

[0020]FIG. 3 shows another example of a switching power amplifier SPA3 according to the invention. In this example the input I3 receives a so-called PCM signal which input is coupled to the input unit INU3. Also in this example the input unit is coupled to the multiphase drive unit MPDU3. Which multiphase drive unit is coupled to the power stage PS3. The output of the power stage is coupled to the summation unit SUMU3. Which summation unit is coupled via a low pass filter LPF3 to the output O3 of the switching power amplifier. In this example instead of a DSD input signal a PCM input signal is used. The input unit INU3 comprises in this example an upsamples and a noise shaper to supply to the multi phase drive unit MPDU3 k signals.

[0021]FIG. 4 shows an example of a switching power amplifier SPA4 as an analog amplifier. In this example the input I4 receives an analog input signal. Which input is coupled to the input unit INU4. In this example the input unit comprises a subtractor SUB4 for subtracting a control signal cs4 from the input signal. It is possible to add a second control signal CS42 from for example the output O4. The output of the subtractor is coupled via a filtering unit FU4 and an AD converter unit ADC4 to the multiphase drive unit MPDU4. The output of the multiphase drive unit is also in this example coupled to the power stage PS4. The output of the power stage is coupled to the summation unit SUMU4. The output of the summation unit is coupled via a low pass filter LPF4 to the output of the switching power amplifier.

[0022] In this example the switching power amplifier SPA4 receives an analogue input signal. When the emphasis is on economy rather than on the ability to process digital audio directly, the advantages of multiphase operation are still significant enough to warrant application in an analog environment. This in done by an analogue control loop, such as used in oversampling low-bit ADC converters, its output feeding into the flash ADC having as many quanta as does the power stage. The output from the ADC may then be processed as outlined before.

[0023] The input of the control loop is the difference found between the input and the output after the summation unit SUMU4. Extra feedback signals may also be taken off at several points on the output filter, including the output.

[0024]FIG. 5 shows an example of a summation unit SUMU5, having a way of summing (avaraging, actually) signals with high precision is to use the so-called autotransformer. The precision of the summation is not influenced by the nature of the core. The signals are supplied via amplifiers A1-A4 to the coils L1 and L2. The coils L1 and L2 are coupled to the output coil L3. It will be noticed that another doubling of the number of phases by adding one level of summation. This method has as one of the great advantages a very high precision of summing the summing network is separate from the output filter the output current produces no magnetization of the cores. The ripple current can be made small. And the size of the coils is independent of the amount of power to be delivered. Further adding the signals in this way the tolerances of the inductors can be neglected.

[0025] Above the invention has been described on the basis of some examples. The man skilled in the art will be well aware of a lot of variations falling within the scope of the invention. 

1. Switching power amplifier comprising an input unit for converting the input signal, a plurality of parallel low power switching power stages, a driving unit to drive stages and a summation unit for summing the output signals of the power stages.
 2. Switching power amplifier as claimed in claim 1 characterized in that the driving unit is a multiphase driving unit.
 3. Switching power amplifier as claimed in claim 1 characterized in that the plurality of parallel low power switching power stages is an N-phase power stage.
 4. Switching power amplifier as claimed in claim 1 characterized in that the summation unit is an inductor summation unit.
 5. Switching power amplifier as claimed in claim 1 characterized in that the input unit comprises a FIR filter and the input signal is a DSD signal.
 6. Switching power amplifier as claimed in claim 1 characterized in that the input unit comprises an upsampler and a noise shaper and the input signal is a PCM signal.
 7. Switching power amplifier as claimed in claim 1 characterized in that the input unit comprises a filter unit and an AD converter and the input signal is an analog signal.
 8. Phase drive unit for use in a switching power amplifier as claimed in claim
 1. 9. Power stage for use in a switching power amplifier as claimed in claim
 1. 10. Inductor summation unit for use in a switching power amplifier as claimed in claim
 1. 11. Method to drive a switched power amplifier. 